ANATOMY AND PHYSIOLOGY OF THE HORSe’s FOOT. 361 
Above the compact tissue of this face the bone is the fibrous 
layer, mixed with very minute and not very numerous carti- 
lage-cells (fig. 10), the fibres being densest nearest this osseous 
surface, into which their extremities are implanted. These 
fibres, as has been noted, generally pass across this surface in 
the same direction as the ligaments from which they are derived, 
and all appear to converge towards the middle or transverse 
ridge of the bone. A large portion of each side of the face, for 
instance, is covered by fibres from the ligaments attaching its 
extremities to the pedal bone, and so on around its upper and 
lower margins; the ridge itself is covered by a thick bundle 
proceeding from the centre of the inferior ligament, and from 
fibres that converge in the middle of its border ; the former 
is particularly strong. So far as I have been able to observe, 
the yellow elastic ligament forming the bourrelet does not 
give any of its fibres for this purpose. This membrane can 
be torn off in pieces by powerful traction of its surrounding 
ligaments, and in points corresponding, of course, to the pas- 
sage of these fibres across and into the face of the bone. 
Very minute blood-vessels are not unfrequently observed to 
pass beneath the membrane for some distance.* 
Nearer the surface the cartilaginous element becomes more 
abundant, and indeed forms a thin and complete layer ; and 
on the surface itself, in very fresh specimens, is a layer of 
pavement epithelium. These coverings appear to be thinnest 
on the middle and lower half of this surface — that on which 
the tendon presses most closely. 
A notable peculiarity in the lower or tendon face of the navi- 
cular bone, and one which, besides its pathological bearings, 
would alone appear to distinguish it from the other two faces, 
covered as they are by ordinary articular cartilage, consists 
in the marked tendency of its fibro-cartilaginous membrane 
to become the seat of calcareous deposition. In examining, 
microscopically, the navicular bones of comparatively young 
horses which had never been suspected of disease in this re- 
gion, I have been surprised to meet with calcareous infiltra- 
tion of the fibrous membrane to a very appreciable degree : 
minute molecules of calcareous matter being interspersed 
among the fibres, or forming small, hard, white, and opaque 
granular masses in the membrane or on the surface of the 
* A casual glance at this surface in a fresh specimen will be sufficient to 
convince the student that it is not covered bj articular cartilage , and a brief 
microscopical examination will prove the structure of the membrane to be 
as I have described it. Besides, it must be sufficiently evident that we 
cannot expect to find articular cartilage where there is no articulation; the 
lower surface of this bone does not articulate with any other bone. 
